Modern information processing systems make extensive use of nonvolatile random access memory devices as mass storage for storing programs and data. The most widely used nonvolatile memory device is hard disks, which are electro-mechanical devices that store data on magnetic material. Recently, semiconductor nonvolatile memory has been developed. One type of semiconductor nonvolatile memory is flash memory which is comprised of a large plurality of floating gate metal oxide silicon field effect transistors arranged as memory cells in typical row and column array. The floating gate design allows information to be retained after power is removed. Flash memory has a number of characteristics which adapt it to be used as mass storage in information processing systems: it is light in weight, occupies very little space, and consumes less power than electro-mechanical devices. Further, it is rugged, and can withstand repeated drops that could destroy electro-mechanical devices.
Flash memory typically contains a plurality of single transistor memory cells which are programmable through hot electron injection and erasable through Fowler-Nordheim tunneling. The programming and erasing of such a memory cell require current to pass through the dielectric surrounding a floating gate electrode. It is found that the dielectric will fail after a certain number of programming and erasing. Because of this property, such types of memory have a finite number of erase-write cycles. Manufacturers of flash cell devices specify the limit for the number erase-write cycles as between 10,000 and 100,000. This is different from other types of memory devices, such as rotating magnetic media (e.g., hard disk) and volatile memory chips (such as dynamic random access memory and static random access memory), which can go through millions of erase-write cycles before failure. As a result, it is desirable to reduce the number of erase-write cycle in flash memory.
Flash memory is typically erased by applying a high voltage to the source terminals of the cells in the memory. Because these source terminals are all connected to one another by metallic busing, the entire memory (or some sub-portions thereof) needs to be erased at the same time. Thus, in an erase operation, valid data along with invalid (dirty) data are erased. This is different from other random access memory devices in which individual bits can be erased and written.
Another difference between flash memory and other types of memory devices is that erase cycles in a flash memory device is slow (when compared to the read-write time of other types of memory devices). This property can significantly reduce the performance of a system utilizing flash memory as its mass storage, unless this system is specially design to compensate for the slow erase cycle.
It can be seen from the above that a new memory architecture needs to be designed so as to effectively use flash memory as mass storage devices.